The present invention relates to manhole closure assemblies and, more particularly, to watertight inserts which prevent storm water from draining into a sanitary sewer through a manhole.
In many urban areas, the increased construction in buildings, streets, and parking lots has significantly decreased the ground surface area which is available for absorbing moisture from rain or snow. Accordingly, this water must be drained off and disposed of through suitable means. In some instances, it is possible to simply collect this water and discharge it into a city's sanitary sewer system. However, sewer systems must be sized such that they can accommodate the maximum influent which can be expected at any time. Accordingly, in those areas receiving a substantial amount of rain, the sewage treatment plant must be increased in size dramatically which results in exceptionally high costs. Thus, it is often advantageous to separately collect and dispose of storm water apart from the sanitary sewer system. This is often done through separate storm sewers.
During rainy weather, an average manhole in a sanitary sewer system can contribute from 3,000 to 12,000 gallons per day of rainwater to the treatment system. Accordingly, in those areas where a separate storm sewer is provided, it has been found advantageous to provide a means for preventing storm water from entering the sanitary sewer system through manholes.
On such system is disclosed in U.S. Pat. No. 4,067,659 issued Jan. 10, 1978. This patent discloses a manhole closure assembly comprising a cover section including a downwardly depressed central portion. A circular mounting flange extends outwardly from the depressed central portion and the flange is adapted to rest upon the inwardly directed manhole cover supporting flange of an annular manhole frame. The downwardly depressed portion of the cover section is depressed in an amount that exceeds the path of spin of a manhole cover resting upon the supporting flange of the annular manhole frame. A first valve member is secured to the cover section to relieve pressure inside the manhole when pressure therein exceeds a predetermined amount. While this patent discloses a system which effectively prevents rainwater from entering the sewer system through the manhole cover, it is plagued by several problems.
For example, since the closure assembly is deep enough to allow the manhole cover to spin freely above the bottom of the closure assembly, the assembly can hold a significant amount of water. This makes it difficult to remove the assembly when it is necessary to enter a manhole. Additionally, when the assembly is formed from a plastic such as polyethylene, it can become softened in hot weather and the weight of any water collected in the assembly can cause it to deform such that it falls into the sewer system through the manhole. The assembly can then travel through the sewer where it can act as a plug if it strikes any object in the sewer system.
Additional manhole disclosure assemblies are disclosed in U.S. Pat. Nos. 3,712,009; 3,798,848; 3,969,847; and 3,973,856. Each of these assemblies comprises a generally planar cover which reduces the amount of water which the assembly will collect. However, each of these closure assemblies requires a separate support ring or flange which is often difficult to install. Additionally, these disclosures are easily damaged when a manhole cover is lifted for removal. The vent valves in the closures often protrude above the surface of the closure and are also easily damaged by the manhole cover.
Accordingly, it would be a significant advancement in the art to provide a means for preventing rainwater from entering a sewer system through a manhole cover which is easy to install and remove. It would be a further advancement to provide such a means which was not easily damaged when a manhole cover is positioned or removed. Such a system is disclosed and claimed herein.